1. Field of Invention
This invention relates to a method and apparatus for deskewing sheets of various sizes while picking the sheets from the top of a stack of documents which are placed in a face up orientation and for depositing the sheets in a document receiving apparatus.
2. Background Information
Sheet feeders of the type to which this invention is directed are commonly called automatic document feeders (ADFs). There are several basic problems associated with ADFs; they are:
1. Document skewing; PA1 2. Documents double feeding; and PA1 3. Stack capacity of the hopper, for example, in which the sheets are stored prior to picking the documents from the stack. PA1 a hopper for storing a stack of documents to be fed; PA1 a picker mechanism for picking a document from said stack and for moving the document downstream in a feeding direction; PA1 a leading edge detector including first and second sensors positioned downstream from said picker mechanism for generating first and second outputs, respectively, as a leading edge of the document encounters said first and second sensors; PA1 first and second drive rollers positioned between said picker mechanism and said first and second sensors; PA1 first and second motors for driving said first and second drive rollers independently of each other; PA1 a document receiver located downstream from said first and second drive rollers in said feeding direction for receiving said document from said first and second drive rollers; and PA1 a controller and a program for receiving said first and second outputs to obtain a measure of skewness of said document approaching said first and second sensors relative to said feeding direction and for controlling the operation of said first and second motors to present said document to said document receiver in a deskewed orientation with respect to said feeding direction. PA1 (a) using independently driven first and second drive wheels for feeding the document so that the leading edge approaches a leading edge detector providing outputs in accordance with the position of the leading edge relative to the feeding direction; PA1 (b) using the outputs from the leading edge detector to energize first and second motors, respectively, driving said first and second drive wheels in a first direction, to enable said leading edge to assume a position perpendicular to said feeding direction prior to the leading edge approaching a receiving member. PA1 (a) applying a predetermined force in a first direction to a first document in the stack of documents; and PA1 (b) subjecting the first document from step (a) to alternating high and low friction forces in a feeding direction which is substantially perpendicular to said first direction.
Document skewing in a sheet feeder is caused by force couples or unbalanced forces that rotate the sheets as they move through the feeder. In other situations, the documents may be skewed, initially, in a stack of documents from which the individual documents are picked.
When the ADFs are used with utilization devices which include character recognition algorithms and compression algorithms, for example, the accuracy and execution times of these algorithms are adversely affected when the documents are skewed relative to a scanner which provides digital images of the documents being scanned.
Many of the existing ADFs lack the capability for mixing documents of different sizes and weights, for example, or they may require constant attention by an operator.
Also, many of the existing ADFs have single drive elements or rollers located in the center of a path along which the documents are driven. The drive elements pick and feed the documents from a point which is located in the center of the stack of documents. If all the sheets are of the same size, the documents being picked from the stack and being moved in a feeding direction are generally driven along the feeding direction, provided the drag forces working on the document are properly balanced. When the stack contains documents of different sizes, the single feed roller in the center of the stack cannot control skewing of the document being fed.
Some of the existing ADFs have moveable stack guides to center the stack in the transport mechanism to feed the documents along the feeding direction. Even when the stack guides house sheets of a single size, some clearance between the stack of sheets and the guides is necessary. When such a clearance exists, some rotation or skewing of the document may exist as it is moved in the feeding direction.
Some prior art ADFs use a pair of sensors to detect document skew. When the skewing becomes excessive, the output of the sensors is used to flag or highlight the skewing for operator intervention, for example.
Jogging the documents in a document stack is one way for reducing the skew of documents being picked from the stack; this is useful when the documents in the stack are all of the same size. When the documents are not all of the same size, the unbalanced forces mentioned work on the document to deskew the document relative to the intended feeding direction.
Double feeding of documents represents the second problem mentioned with ADFs. Double feeding of documents from a document stack is a serious problem because part or all of the data on the second or unintended document being fed may be lost. Generally, it is easier to feed documents of the same size and thickness than those of varying sizes and thicknesses. When the thicknesses of documents within the stack vary over a narrow range of paper thicknesses, say 0.004 inch (20#) to 0.006 inch (24#), it is not as difficult as feeding documents over a wide range of paper thicknesses ranging from 0.003 inch (14#) to 0.012 inch (100#).
In general, retard rollers are used to minimize the double feeding mentioned. On some equipment, an operator adjusts the pressure on the retard rollers when there is a change in the-thickness of a batch of documents to be processed. The effectiveness of such operator adjustments is generally limited to a paper thickness falling in a small range, typically +/-0.002 inch.
The third problem mentioned with regard to ADFs relates to the storage capacity of the hopper in which the documents are stored prior to feeding. In this regard, the capacity of fixed inclined tray or platform feeders is generally limited to 100 sheets maximum. Most tray feeders generally specify a maximum of 50 sheets. When loading this type of hopper, the operator must fan the stack of sheets so that the top sheet is ahead of the second sheet, and the second sheet is ahead of the third sheet etc. By shingling or cascading the sheets in this manner, the ends of the sheets are set up so that the top sheet feeds first, followed by the sequence of the remaining sheets. Obviously, if the sheets are not fanned properly, double feeds or misfeeds can occur.
In order to feed from the top of a stack of several hundred documents with the sheet picker located near the top of the stack, either the stack has to be raised to keep the "top sheets" against the picker, or the picker must be lowered to reach the top sheet in a stack which decreases in height as documents are picked from the stack. Large capacity sheet feeders (250 sheets and up) which are used with image scanners, typically use horizontal trays which move up and down in an elevator-like manner. The elevator type feeders are complex and expensive to maintain because linear bearings are required to guide the associated horizontal platform on which the stack of sheets is located.